Ceramic materials have enjoyed great success as igniters in gas-fired furnaces, stoves and clothes dryers. A ceramic igniter typically includes a ceramic hot surface element having a hairpin or U-shape which contains two conductive end portions and a highly resistive middle portion. When the element ends are connected to electrified leads, the highly resistive middle portion (or "hot zone") rises in temperature.
Since these igniters are resistively heated, each of its ends must be electrically connected to a conductive lead, typically a copper wire lead. However, the problems associated with connecting the ceramic hot surface element ends to leads are well known. One common problem is that the ceramic material and the lead wire do not bond well together. EP 0486009 ("Miller") discloses and FIG. 2 herein presents a conventional igniter system in which a braze 91 is applied to an end of the conductive ceramic 92 by brushing, the braze is then vacuum fired, solder 93 is applied to the braze, and the lead wire 95 is attached to the solder. This solder is typically applied by carefully directing a high temperature (1600-1800.degree. C.) flame upon the end of the brazed ceramic leg, contacting the solder to the hot leg (thereby causing the solder to flow) and then placing the lead wire in the still-liquid solder.
However, the use of solder as described above often causes a host of problems for this technology. First, this method is a very sensitive and time-consuming operation. Second, exposing the hot surface element to the high temperature flame often causes a crack in the igniter. The crack may be due to a coefficient of thermal expansion ("CTE") mismatch between the ceramic hot surface element ("CHSE") and either the solder or the braze. It may also be due to thermal shock of the CHSE. Third, even if the soldering is successful, the resultant solder coverage is typically only on one side of the leg (as shown in FIG. 2). If excessive pressure is applied to the tip of the wire (which occurs frequently in the cementing operation), the lead wire can be pulled free from its bond. Fourth, it is known that, in use, solder is highly susceptible to oxidation and is so frequently the cause of premature aging of the igniter.
Some investigators have tried to manage the problems caused by solder. For example, U.S. Pat. No. 5,564,618 ("Axelson") recognized that the CTE mismatch between the braze and the solder was causing breakage during the soldering step, and sought to minimize the braze by using a silk screening approach. Although this method eliminated some cracks during the soldering step, the three other problems described above remained. Moreover, the small amount of braze necessitated by the Axelson method insured a substantially weaker bond between the braze, solder and wire, leading to substantial failures during pull-off testing.
In another attempt to solve this CTE mismatch problem, the legs of the ceramic hot surface element are dipped into a braze reservoir and then vacuum fired to cure the braze. Ideally, this method should provide a full and even 360 degree coating of the leg, so that the curing of the braze puts the leg into desirable compression. However, because the dipping procedure is inexact, there is typically a large variation in both the coating thickness and the area of coverage, resulting in an undesirable stress distribution. Moreover, the three other problems caused by the use of solder still remain. Lastly, this process uses a large amount of very expensive braze.
Some investigators have tried to eliminate solder from ceramic igniter termination systems. For example, GB 2,095,959 discloses a ceramic block which provides mechanical stability to the hot surface element-wire system. Nichrome wires are physically placed into machined holes or grooves in the hot surface element, and the wires are mechanically held in place by a metallic overlayer which can be either flame-sprayed, galvanized (i.e., plated), or fritted (glass), or nichrome or silver coated. Terminals are attached to the lead wires, and insulation grips are attached to the lead wires. A feature in the block accepts the insulation grips on the wires. The redundancy of mechanical support embodied in the full ceramic block/extensive groove/grip system of GB '959 indicates that this inventor was very concerned that the lead wires would break free from the hot surface element and cause the system to fail.
U.S. Pat. No. 5,804,092 ("Salzer") teaches a modular ceramic igniter system, in which the ceramic hot surface element is plugged into a socket having a conductive contact therein. In some embodiments, these sockets have spring-like contacts which help hold the leg of the ceramic igniter in place. In others, the sockets are tube-shaped. However, each of these systems are plug-in systems which are designed to be temporary and therefore easily pulled apart.
In sum, the use of solder in ceramic igniter systems has caused a host of both process and performance problems. Attempts to design systems which eliminate solder have resulted in either fragile or temporary systems. Therefore, there is a need for a ceramic igniter system having a permanent, solderless electrical connection for ceramic hot surface elements.